Heretofore, in an oil-based ballpoint pen, a drawback is liable to occur in which, when a solvent of an ink at a tip end portion of a ball tip is evaporated and, then, the ink becomes thickened, the ink is not easily ejected at the start of writing to cause blobbing of written letters, or a written letter with a sufficient density cannot be obtained until the ball starts fully rotating. Further, this blobbing phenomenon greatly depends on environmental conditions and often occurs under low-temperature or high-temperature conditions to cause distress. Still further, in a case of using a solvent having a high volatility, as defined in the Claims, this phenomenon occurs outstandingly and becomes a serious problem. In order to solve these problems, various efforts have heretofore been exerted. For example, in an invention according to Japanese Examined Patent Publication (Kokoku) No. 61-52872, the invention in which fluidity of an ink is maintained by adding a specific nonionic surfactant is described; in Japanese Examined Patent Publication (Kokoku) No. 57-38629, an invention of using a high boiling point aromatic hydrocarbon as a solvent for suppressing deterioration of an ink to be caused by drying or by absorbing moisture is described; in an invention according to Japanese Unexamined Patent Publication (Kokai) No. 3-28279, fluidity of an ink is maintained by adding a phosphoric acid ester; in an invention according to Japanese Unexamined Patent Publication (Kokai) No. 6-247093, an ink is prevented from being completely dried out by using a nonvolatile solvent.
In an invention according to Japanese Unexamined Patent Publication (Kokai) No. 11-158421, by compounding a salt of a basic dye with a phosphoric acid ester, crystallization of a dye at a pen tip is suppressed and the ink is allowed to be in a state of from a grease to a paste such that the ink is hard to dry-solidify at a tip end portion of a pen, whereby the fluidity of an ink is maintained and, as a result, smooth writing at the start of writing is secured, to thereby improve the problematic blobbing phenomenon. Also, in an invention according to Japanese Unexamined Patent Publication (Kokai) No. 11-21495, the same effect as described above is generated by neutralizing an acidic polyoxyethylene alkyl ether phosphoric acid ester with a predetermined alkyl alkanolamine or morpholine. However, the amine substance used on this occasion has a strong odor and high reactivity and, therefore, a degree of freedom of selection of solvents, dyes and the like is narrow. Further, there is no measure for determining a stable neutralized material.
However, although there scarcely arises a problem in a solvent having such a vapor pressure of less than 0.01 mmHg as used in a conventional oil-based ballpoint pen, in the case of using a solvent having a high vapor pressure, when an ink agglomerate is solidified in a periphery of a ball, an extremely high writing load is required to start writing in which the ball moves.
An object of the present invention is to provide an oil-based ink composition for a ballpoint pen which, unlike conventional techniques, suppresses blobbing of a written letter at the start of writing and is excellent in soft smooth writing at the start of writing, and a ballpoint pen using the ink composition.